Method of treating an underground formation to prevent liquid loss to large cavities in a formation



I. HAVENAAR 3,347,316

REVENT LIQUID Oct. 17, 1967 METHOD OF TREATING AN UNDERGROUND FORMATION TO P LOSS TO LARGE CAVITIES IN A FORMATION Filed Oct. 26. 1964 FIG.

FIG. 2

FIG. 3

FIG. 4

INVENTORI IZAAK HAVENAAR 511% Q HIS AGENT United States Patent ()fiice 3,347,315 Patented 'Oct. 17, 1967 3,347,316 METHUD OF TREATHNG AN UNDERGRQUND FORMATION T PREVENT LIQUID LGSS TU LARGE CAVITEES IN A FGRMATEfiN Izaak Havenaar, Rijswijk, Netherlands, assiguor to Shell Oii Company, New Yorlr, N.Y., a corporation of Delaware Filed Oct. 26, 1964, Ser. No. 466,259 3 Claims. (Cl. 166-29) ABSTRACT OF THE DISCLOSURE A method of preventing fluid loss into large cavities in an underground formation in communication with a borehole comprising injecting into the cavities a liquid containing particles having a specific gravity greater than that of the liquid and having specific dimensions so as to allow the particles to deposit in the cavities and thereafter injecting a liquid having plastering properties.

The invention relates to a method of treating an underground formation to prevent liquid loss from a hole penetrating the underground formation.

Such liquid losses may occur in wells or holes penetrating into an earth formation, in particular when carrying out a process whereby the hole is being drilled there in. On passing very permeable, cavernous or fractured rocks, the interior of the hole comes into communication with large-volume spaces within the rock, as a result of which the liquid present in the hole is drained from the hole into the spaces.

These liquid losses into an underground cavity or cavity system consisting of a plurality of cavities communicating with each other, can often be combatted by adding plugging material to the liquid in the hole. Such plugging material (e.g., fibers, flakes, sawdust, shredded wood) has preferably a size greater than the openings existing between the hole and the cavity system, such that the plugging material is filtered out in the openings when the liquid passes out of the hole into the cavities. The filtered out material plugs the openings, thereby decreasing the passage therethrough to such an extent that the plastering material added to the liquid seals the passage through the pores left between the plugging material.

As the drilling liquid used for drilling holes in a permeable formation has already the required plastering properties, it will be sufficient to add plugging material to the drilling liquid when the loss of liquid rises to an unacceptable value.

Another way to obstruct the passage between the hole and the cavity is by injecting a substance (such as cement) which when in the cavity, will set into a solid mass.

However, the above methods fail when used to attempt the prevention of leakage of liquid when the holes are in communication with cavity systems via large-sized openings.

It has also already been proposed to stop the flow of liquid out of the hole into cavities by introducing into the circulation fluid successive slugs of gravel, which slugs are of progessively decreasing particle and each of which has mixed therewith a sealing composition. The particle size runs from /2 for the first batch of aggregate to Ms, and the method is based on the fact that the largest particles will become lodged in the cavity and form a primary frame work on which later smaller particles may be deposited. It will be clear that by this method it will only be possible to seal those crevices which have a passage which is not much larger than the largest dimension of the particles used.

sand and gravel, whereafter the side of the sand body' facing the bore-hole is provided with a filter cake of diatomaceous earth, on which filter cake there is deposited a mud sheath produced by the circulation fluid. It will be clear, that if the crevices or cavities communicating with the bore-hole have a very large volume, large amounts of money and time will be involved before these volumes have been filled up by sand and gravel.

In still another way, it has already been proposed to use sand mixed with coarse ground dehydrated expansibletype bentonite clay, which is mixed with water and injected into the cavity or cavity system to be plugged. This method cannot, however, seal cavities having a large-sized cross-section as the clay/sand mixture will not bridge the distance between the side walls of the cavities and consequently on prolonged injection will be pushed deeply into the formation.

The object of the present invention is a method suitable for preventing the loss of fluid out of a borehole penetrating a formation either permeable or impermeable, into cavities or crevices communicating with the borehole, wherein the cross-section of the entrances to the cavities or crevices as well as the width of the cavities or crevices may be very large. At the same time the method will also be suitable when being applied in a borehole for sealing such large cavities, to seal those cavities or fissures which are of minor dimensions.

According to the invention, the method comprises the steps of injecting into at least one of the cavities communicating with the borehole, a suspension of particles in a liquid, flowing the suspension through the cavity, readily separating particles out of the suspension by gravity, depositing these separated particles in the cavity, stopping the injection of the suspension into the cavity after the passage through the cavity has been blocked by the particles separated from the suspension, and injecting into the cavity a liquid having plastering properties.

As the suspension has the property of readily separating the particles from the liquid, the particles will be deposited at those spots where the velocity of the injected liquid has dropped to a sufficiently low value. The amount of sand deposited will gradually grow and at last fill up the passage through the cavity at this spot. On the side of the mass of sand facing the borehole, there is subsequently deposited a sealing sheath by injecting a fluid having plastering properties. It will be preferred to use angular particles as then a stronger barrier or obstruction is obtained in the cavity than with round particles.

The above-mentioned property of readily separating by gravity the particles from the suspension to be injected into the cavities or crevices is not known from the prior art. Preferably the suspension according to the invention is formed by water mixed with sand or gravel. If it is considered desirable to add other substances to this suspension, care has to be taken that the property of readily separating the particles from the liquid is not spoiled thereby. Although a gel strength of 10 dynes per square centimeter is allowable, it is advised to use a suspension having a smaller gel strength, preferably between 0 and 2 dynes per square centimeter.

Preferably at least 50% of the particles suspended in the liquid are coarser than 140 mesh but finer than 12 mesh.- The best results, however, will be obtained by a suspension in which at least of the particles lies within the above-mentioned size range. Hereby the greater part of the particles suspended in the liquid will be deposited at the same spot, which decreases the amount of suspension to be injected and speeds up the operation.

Most preferably an amount of at least 50% but preferably at least 90% of the particles to be used in the suspension is finer than 18 mesh but coarser than 140 mesh.

It is advisable to use large-size particles where a suspension of high gel strength (but preferably lower than 10 dynes per square centimeter) is being used.

The invention may be carried into practice in various ways but one specific embodiment will now be described with reference to the accompanying drawings in which:

FIGURES 1, 2, 3 and 4 schematically show sequential steps by which a cavity system in a formation (taken in cross section) can be plugged off by means of the method according to the invention.

In FIGURE 1 there is shown a longitudinal section through a borehole 1 passing through a permeable formation 2. So as to prevent the loss of liquid into the pore space of the formation 2 which in the example as given is of a permeable nature, the drilling liquid used in the hole 1 has plastering properties. To this end a material having plastering properties, such as finely divided clay may be added to the liquid, which clay when the liquid passes into the formation pore space, is filtered from the liquid and left on the wall of the borehole 1 in the form of a mud cake 3. Since the resistance to flow through the mud cake is rather high, the further flow of liquid out of the borehole ll into the pore space of the formation 2 is decreased and nearly the full pressure difference existing between the interior of the borehole and the pore space of the formation 2 prevails over the mud cake 3.

However, the drill bit on its way downwardly into the formation 2 has passed a cavity 4 which forms part of a (not shown) cavity system of large volume and extending over a large distance.

The drilling liquid present in the borehole 1 passes out of the borehole 1 into the cavity 4 of which only part has been shown in the drawing. As the entrance 5 to the cavity 4 has dimensions which are a great many times larger than the dimensions of the plastering material present in the drilling liquid, this material is not able to seal this entrance to prevent the liquid from entering the cavity. Even larger-sized material such as plugging material in the form of fibers or flakes is not able to bridge the entrance 5 to the cavity 4 and is lost together with the drilling liquid in the cavity system 4.

As the cavity system is of very large size, the permeable area of the formation 2 bounding the cavity system 4 is also of a very large size, which makes sealing thereof by the plastering material present in the liquid flowing into the cavity 4 impossible.

When the large loss of liquid into the cavity 4 is discovered the drill string (not shown) is preferably retracted from the hole and the drill bit removed therefrom, whereafter the drill string (or, if desired, another pipe), is lowered into the hole until the lower end of the pipe is located at some distance above the entrance 5. If bits with large-size mud outflow openings are being used, removal of the bit may be omitted.

The level of the entrance 5 can be determined in various ways, e.g., by lowering a flow meter in the hole which will show a certain flow-velocity of the liquid just above the opening 5 and a flow-velocity of zero or nearly zero just below the opening 5. In another way the level of the opening 5 may be detected by lowering a television camera or a photographic camera for visual inspection of the wall of the hole 1. An indication of the depth at which the opening 5 occurs can also be obtained by observing the loss of drilling fluid into the formation. A strong increase in this loss is an indication that a cavity or cavity system has been contacted, probably at the bottom of the hole.

Once the lower end of the pipe (not shown) is positioned at a level above the entrance 5, the top of the pipe is brought into communication with a supply of liquid having particles suspended therein. If desired, the

4;. suspension to be injected can be made before the pressure thereof is raised to the injection pressure. In another way the liquid may first be pressurized to the desired value whereafter the particles are added thereto.

The particles may be formed, e.g., by sand, gravel, barite, or iron-ores such as ilmenite or magnetite. These particles should have a specific gravity greater than the specific gravity of the liquid in which they are suspended. Nat-er is preferably used as the liquid, although other liquids, such as oil, may be used as well.

Preferably there is mixed to the liquid an amount of particles of which preferably at least thereof lies within the range of 12 (preferably 18) and mesh.

The dimensions of the 12, 18 and 140 mesh sieves will be given below:

If it should be considered desirable to add other substances to the liquid than the particles used for building up the obstruction within the cavity, care should be taken that the gel strength of the suspension does not exceed 10 dynes per square centimeter. It is advised, in order to speed up the operation to use a suspension having a gel strength as low as possible, e.g., lower than 2 and preferably zero or nearly zero.

Although water, either fresh or salt, is generally the cheapest liquid to be used for forming the liquid phase of the suspension, the invention is not limited thereto. Other liquids can be used as well, although the Choice is for practical reasons limited to oil only.

The suspension of particles in the liquid will preferably have a concentration of at least 200 kilograms particles per cubic meter of liquid. Lower concentrations, which may also be used, require a greater amount of liquid for placing the same amount of particles in the cavity.

The particles suspended in the liquid are pumped through the pipe into the hole, and the flow thereof is, due to the pressure difference existing between the hole 1 and the cavity 4, directed into the cavity 4 (vide arrow 6 in FIGURE 2). In the cavity 4, the particles separated from the liquid are deposited, until an increase of the injection pressure indicates that the passage through the cavity 4 has been closed off by a mass of particles 7 (vide FIGURE 3).

Then the particles of the suspension which is still present in the injection pipe, will, as the flow of liquid into the cavity has stopped, start to separate from the suspension and settle in the bottom part of the hole 1. Before drilling can be resumed, these particles have to be removed, which can be achieved, e.g., by circulating liquid through a pipe and the annular space surrounding the pipe, and lowering the pipe in the hole such that the lower end of the pipe is always just above the column of particles which is to be removed from the bottom part of the hole.

If drilling liquid is used for circulating away the particles, some of the liquid will enter formation 2 via the permeable wall of the cavity 4, as well as of the mass of sand 7, thereby forming a mud cake 8 on the wall of the cavity 4 and on the mass of particles 7 obstructing the passage through the cavity. As the mud cake 8 forms a continuation of the mud cake 3 which was partly already present on the wall of the borehole 1 and partly deposited later on, the communication between the borehole and the formation pore space is broken and no further liquid will escape from the borehole. If the formation 2 has an impermeable nature, this mud cake will only be deposited on the side of the mass 7 facing the borehole 1.

To decrease the amount of particles which has to be removed from the bottom part of the hole, the suspension still present in the injection pipe on the moment that the passage through the cavity is closed off, can be returned to the top of the well by injecting liquid into the top of the annular space surrounding the pipe, and opening the top end of the injection pipe. In this way the direction of flow of the suspension is reversed and the suspension is removed from the hole. If the liquid injected into the annular space is a drilling liquid, the desired mud cake 8 in the cavity 4 (vide FIGURE 4) will be formed simultaneously with the removal of the suspension from the injection pipe. If this liquid has no plastering properties, a liquid having such properties will be injected into the cavity after the removal of the suspension out of the injection pipe.

The rate at which the suspension of particles in water is injected into the borehole 1 is preferably at least 50 cubic meters per hour. It will be clear that lower rates may be used, but that then the danger exists of the particles settling in the borehole, as a result of which the liquid entering the cavity 4 will not carry any particles, or only a very limited amount of particles.

In some cases it may be advantageous to lower the injection rate (even below the above-mentioned minimum value of 50 cubic meters of suspension per hour) when an indication is obtained that the cross-section of the cavity is diminishing at a certain point due to the settling of the particles therein. Such may, e.g., be ascertained by measuring the injection pressure. By changing the injection rate to a lower value, the rate at which the passage is being closed will increase.

After the injection of the plastering liquid into the hole 1 and the cavity 4, the pressure in the hole may be increased so as to test the strength of the obstruction 7 in the cavity 4 to withstand a difference in pressure existing over said obstruction. If the obstruction does not hold, the whole process can be carried out anew without taking any special measure.

As angular particles will, when settling, form a stronger barrier or obstruction than round particles the use of the first type of particles is to be preferred when applying the method according to the invention.

It will be clear that the introduction of the suspension of particles into the borehole need not necessarily take place via an injection pipe suspended from the top of the hole. If desired the introduction may take place via the casing. However, if the borehole penetrates into a permeable formation, the passage of the suspension of particles through unlined parts of the hole is not recommended, as the particles present in the suspension will abrade the plastering layer present on the wall of the hole, which sometimes may result in an increase of liquid loss.

Settling of particles in the lower part of the borehole 1 can be prevented by placing a packer just below the entrance 5 whereby the passage in the hole 1 is obstructed and the total amount of particles injected into the hole 1 will pass into the cavity 4 via the entrance 5.

Although the present invention has been illustrated by a description thereof when applied in a hole penetrating into a permeable formation, it is not limited thereto. With equally favorable results the invention may be applied in a hole pentrating into an impermeable formation.

It will further be clear that the present invention is not limited to the examples given above, but that without departing from the spirit of the invention various types of particles, various types of liquids for suspending the particles therein, and various types of plastering materials can be used for carrying out the method according to the invention.

I claim as my invention:

1. A method of treating an underground formation to prevent liquid loss into large cavities in communication with a borehole drilled into the formation comprising;

(a) injecting through a tubing string in the borehole and into the large cavities water containing suspended sand particles, at least 50% of said particles being coarser than mesh but finer than 12 mesh and having a specific gravity greater than that of Water and having a gel strength between 0 and 2 dynes per square centimeter, at a flow rate such that the sand particles separate out of the Water by gravity and deposit in the large cavities;

(b) stopping the injection of the water containing the sand particles after the cavities and passages thereto have been blocked; and

(c) injecting into the blocked cavities a liquid having plastering properties.

2. The method of claim 1 wherein the suspension has a concentration of 200 kilograms of particles per cubic meter of liquid.

3. The method of claim 1 wherein the suspension is introduced into the Well at a pumping rate of at least 5 0 cubic meters per hour.

References Cited UNITED STATES PATENTS 2,041,086 5/1936 OBrien 166 29 2,626,779 1/ 1953 Armentrout -72 X 2,648,522 8/1953 Armentrout 175-72 2,693,854 11/1954 Abendroth 166-42 2,812,161 11/1957 Mayhew 175-72 X 2,815,079 12/1957 Goins et al. 166-29 2,943,679 7/1960 Scott et al 166-21 3,219,111 11/1965 Armentrout 17572 X 3,280,912 10/1966 Sheffield 166-29 OTHER REFERENCES Hodgman, Charles D. et al., Handbook of Chemistry and Physics, 36th ed., Cleveland, Ohio, Chemical Rubber Pub. Co. 1954, pp. 3078 and 3079.

Gionta, Al, et al., Plastic Foils Thief Zones. In World Oil, December 1955, pp. 138 and 140.

CHARLES E. OCONNELL, Primary Examiner. JACOB L. NACKENOFF, Examiner.

I. A. CALVERT, Assistant Examiner. 

1. A METHOD OF TREATING AN UNDERGROUND FORMATION TO PREVENT LIQUID LOSS INTO LARGE CAVITIES IN COMMUNICATION WITH A BOREHOLE DRILLED INTO THE FORMATION COMPRISING; (A) INJECTING THROUGH A TUBING STRING IN THE BOREHOLE AND INTO THE LARGE CAVITIES WATER CONTAINING SUSPENDED SAND PARTICLES, AT LEAST 50% OF SAID PARTICLES BEING COARSER THAN 140 MESH BUT FINER THAN 12 MESH AND HAVING A SPECIFIC GRAVITY GREATER THAN THAT OF WATER AND HAVING A GEL STRENGTH BETWEEN 0 AND 2 DYNES PER SQUARE CENTIMETER, AT A FLOW RATE SUCH THAT THE SAND PARTICLES SEPARATE OUT OF THE WATER BY GRAVITY AND DEPOSIT IN THE LARGE CAVITIES; (B) STOPPING THE INJECTION OF THE WATER CONTAINING THE SAND PARTICLES AFTER THE CAVITIES AND PASSAGES THERETO HAVE BEEN BLOCKED; AND (C) INJECTING INTO THE BLOCKED CAVITIES A LIQUID HAVING PLASTERING PROPERTIES. 